Method for mitigating passive intermodulation

ABSTRACT

Materials and methods for mitigating passive intermodulation. A membrane for reducing passive intermodulation includes a first polymeric layer, a second polymeric layer, and a continuous metal layer encapsulated between the first and second polymeric layers. A self-adhesive radio frequency barrier tape includes a waterproof polymeric top layer, a metal-containing layer adhered by an adhesive layer to the polymeric top layer, a pressure sensitive adhesive layer adhered to the metal-containing layer, and a release liner on a bottom surface of the pressure sensitive adhesive layer. A method of mitigating passive intermodulation includes passing a probe over an area of interest, the probe being sensitive to an intermodulation frequency of interest, and identifying a suspected source of passive intermodulation when the amplitude of the probe output exceeds a threshold at the frequency of interest. The method further includes covering the suspected passive intermodulation source with a radio frequency barrier material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/786,785, filed Oct. 18, 2017 and titled “Method for MitigatingPassive Intermodulation”, and this application claims the benefit ofprovisional U.S. Patent Application No. 62/426,673, filed Nov. 28, 2016and titled “PIM Reducing RF Barrier”, the entire disclosures of whichare hereby incorporated by reference herein for all purposes.

BACKGROUND

An essential element of modern mobile communications systems is the“cell site.” The cell site includes one or more directional base stationantennas aimed at a desired geographical area of coverage with coaxialcable connecting the antennas to base station radio equipment. The basestation radio equipment acts as a transceiver, transmitting high powersignals (37 dBm to 49 dBm channel power) in the direction of mobileusers and receives low power signals (as low as −106 dBm channel power)from mobile user equipment. Often the range of a cell site is uplinklimited, meaning that the distance that a particular cell site is ableto cover is limited by the transmit power of the mobile equipment andthe path loss between the mobile equipment and the cell site. The signalreceived at the cell site from mobile equipment reduces by 6 dB eachtime the distance between the mobile equipment and the cell site isdoubled. Eventually, the signal from the mobile equipment drops to alevel where the base station receiver is not able to distinguish thesignal from background noise, and the call is dropped.

Mobile operators typically design their network of cell sites to achievea signal-to-interference plus noise ratio (SINR) of >6 dB across theregion of coverage. This means that the signal level arriving frommobile equipment at each base station receiver should be at least 6 dBhigher than all sources of noise generated within or received by thebase station receiver. For data networks, the network of cell sites maybe designed to achieve even higher SINR levels to enable faster datatransmission.

In order to achieve the strongest possible signal from mobile equipment,it is desirable to locate the cell site antennas at an elevation that ishigher than the average clutter (trees, buildings, etc.) in the area tobe covered. This can be achieved by erecting a tower at the cell site tosupport the antennas at the desired elevation or can be achieved bymounting the antennas on existing infrastructure, such as buildingrooftops. When mounting antennas on buildings, the building owner mayrequire for aesthetic reasons that the operator only install theantennas at locations that are not visible from the ground outside thebuilding. This often forces the antennas to be installed near the centerof the building rooftop rather than near the edges of the building.

For example, FIG. 1 shows a ground-level perspective view of a building100, showing that no cell site is visible. However, FIG. 2 showsbuilding 100 from an upper oblique view, revealing cell site 200 on roof201, set back from the edges of roof 201.

The performance of cell sites installed on building rooftops is oftenlimited due to passive intermodulation (PIM). Passive intermodulationoccurs when the high-power downlink signals broadcast by the basestation antennas mix at passive, non-linear junctions in the RF path,creating new signals. If these new signals (intermodulation products)fall in an operator's uplink band, they act as interference and reduceSINR. As the SINR is reduced, the geographic coverage of the cell siteis reduced and the data capacity of that cell site is reduced.

BRIEF SUMMARY

According to one aspect, a method of mitigating passive intermodulationcomprises injecting two signals of two known frequencies into an antennasystem, such that the antenna system produces radio frequency radiationcontaining the two known frequencies directed toward an area to betested for passive intermodulation. The method further comprisescalculating an intermodulation frequency from the two known frequencies,and connecting a probe to a spectrum analyzer. The spectrum analyzer isconfigured to receive radio frequency signals produced at theintermodulation frequency. The method further comprises passing theprobe over the area to be tested while simultaneously monitoring thesignal received by the spectrum analyzer, and identifying a suspectedsource of passive intermodulation when the spectrum analyzer detects asignal amplitude above a threshold value at the intermodulationfrequency. The method further comprises covering the suspected source ofpassive intermodulation with a temporary radio frequency barriermaterial configured to mitigate passive intermodulation, andre-measuring the signal amplitude at the suspected source of passiveintermodulation at the intermodulation frequency with the radiofrequency barrier material in place, to verify a reduction in passiveintermodulation. The method further comprises replacing the temporaryradio frequency barrier material with a permanent radio frequencybarrier material. In some embodiments, the permanent radio frequencybarrier material comprises a first polymeric layer, a second polymericlayer, and a metal layer encapsulated between the first and secondpolymeric layers. In some embodiments, the metal layer is continuous. Insome embodiments, the metal layer is perforated. In some embodiments,the metal layer is deposited on a plastic sheet, and the metal layer andthe plastic sheet are encapsulated between the first and secondpolymeric layers. In some embodiments, at least one of the first andsecond polymeric layers comprises thermoplastic polyolefin (TPO). Insome embodiments, at least one of the first and second polymeric layerscomprises ethylene propylene diene monomer (EPDM). In some embodiments,at least one of the first and second polymeric layers comprisespolyvinyl chloride (PVC). In some embodiments, at least one of the firstand second polymeric layers comprises modified bitumen. In someembodiments, the area to be tested for passive intermodulation is on arooftop, and covering the suspected source of passive intermodulationwith a permanent radio frequency barrier material comprises covering thesuspected source of passive intermodulation with a walk pad. In someembodiments, the area to be tested for passive intermodulation is on arooftop, and covering the suspected source of passive intermodulationwith a permanent radio frequency barrier material comprises coveringmost or all of the area to be tested with a roofing membrane configuredto be a radio frequency barrier. In some embodiments, covering thesuspected source of passive intermodulation with a permanent radiofrequency barrier material comprises covering the suspected source ofpassive intermodulation with a self-adhesive radio frequency barriermaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a ground-level perspective view of a building.

FIG. 2 shows the building of FIG. 1 from an upper oblique view,revealing a cell site on the roof of the building.

FIG. 3 illustrates the generation of passive intermodulation.

FIG. 4 illustrates the building of FIG. 1 with an RF barrier installedon the roof of the building, in accordance with embodiments of theinvention.

FIGS. 5 and 6 show measurements of a typical cell site installation,before and after placement of an RF barrier in accordance withembodiments of the invention.

FIG. 7 illustrates a PIM barrier membrane, in accordance withembodiments of the invention.

FIG. 8 illustrates a process of making a PIM barrier membrane, inaccordance with embodiments of the invention.

FIG. 9 illustrates a PIM barrier membrane in accordance with otherembodiments of the invention.

FIG. 10 illustrates an exploded view of a self-adhesive RF barrier tapein accordance with embodiments of the invention.

FIG. 11 shows an edge view of the self-adhesive RF barrier tape of FIG.10, in its assembled state.

FIG. 12 illustrates a technique for locating PIM sources, in accordancewith embodiments of the invention.

FIG. 13 shows suspected PIM sources covered with respective pieces ofbarrier material, in accordance with embodiments of the invention.

FIG. 14 illustrates large pieces of barrier material applied to theentire area of interest on a roof, in accordance with embodiments of theinvention.

FIG. 15 illustrates a mat RF barrier, in accordance with embodiments ofthe invention.

FIG. 16 illustrates the installation of the mat RF barrier of FIG. 15,in accordance with embodiments of the invention.

FIG. 17 illustrates a mat RF barrier in accordance with otherembodiments of the invention.

FIG. 18 illustrates an RF barrier membrane in accordance with otherembodiments of the invention.

FIG. 19 illustrates the installation of the RF barrier membrane of FIG.18, in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Loosely touching metal-to-metal surfaces as well as rusty metal surfacesbehave in a non-linear fashion and can generate PIM when illuminatedwith RF energy. On rooftop cell site installations where the antennasare placed near the center of the roof, many non-linear objects may bepresent in front of the antenna. PIM sources may be located within thecellular network equipment or externally. Common external sourcesinclude loosely contacting sheet metal flashing used forweatherproofing, loose connections at overlapping metal decking members,rusty bolts, vent pipes, and the like.

Common internal sources may include loose mechanical junctions, oxidizedor contaminated surfaces, connections or contact between dissimilarmetals, and the like. Loose metal-to-metal contacts can also occur atthe fastener locations, where due to time and thermal expansion, theheads of screws or nails are no longer applying high contact pressure.Since steel is a common material used in roof construction, rust islikely to form at locations where moisture has been able to penetratethe roof membrane.

FIG. 3 shows an enlarged view of a portion of FIG. 2, and illustratesPIM in more detail. Cell site 200 includes one or more antennas 301,connected to control electronics 302. Cell site 200 may be surrounded byan RF-transparent screen 303 for aesthetic purposes. Antenna 301transmits relatively high-power downlink signals 304, and receivesrelatively low-power uplink signals 305 from a number of cellular phonesin the vicinity.

In FIG. 3, various PIM sources 306 are shown emitting RF noise 307 inresponse to exposure to downlink signals 304. Some of the RF noise isdirected toward antenna 301, and some may fall within the uplink band,thus reducing the uplink SINR.

Many PIM sources 306 may be encompassed in the relatively wide area ofroof 201 exposed between cell site 200 and the edges of roof 201 andsubject to downlink signals 304.

Rooftop PIM sources at cellular installations have been shown to produce3rd order intermodulation products (IM3) as high as −50 dBm when thesite is tested by injecting 43 dBm test tones into the cell siteantenna. This level of PIM is almost 50 dB (100,000 times) higher thanthe typical “passing” PIM level required for acceptable siteperformance.

In accordance with embodiments of the invention, covering the non-linearobjects on the rooftop with an RF barrier significantly reduces the PIMgenerated by these non-linear objects. For every 1 dB that the RFbarrier is able to attenuate the signal arriving at a non-linearjunction, the IM3 level seen at the base station receiver is typicallyreduced by 3.5 dB.

FIG. 4 illustrates building 100 and cell site 200 with an RF barrier 401installed on roof 201, over PIM sources 306. RF barrier 401 reduces theRF energy reaching PIM sources 306, and consequently reduces the amountof PIM generated at sources 306.

RF barrier 401 preferably reduces the RF energy reaching PIM sources 306by at least 10 dB. This will reduce the magnitude of PIM generated byPIM sources under the barrier by approximately 35 dB. In otherembodiments more or less attenuation may be provided. For example, FIGS.5 and 6 show measurements of a typical cell site installation, beforeand after placement of an RF barrier such as RF barrier 401. In theexample of FIGS. 5 and 6, a reduction in PIM magnitude of approximately36 dB was achieved, from −61.1 dBm to −97.8 dBm.

The attenuation may be achieved through absorption, reflection or acombination of the two. For example, a reflective material such as RFbarrier 401 reflects at least some of the original RF signals 304 fromantenna 301, reducing the induced current in the underlying PIM sources,and ultimately reducing the magnitude of the PIM generated.

Preferably, the materials used to construct RF barrier 401 do notthemselves become a PIM source. For example, RF barrier 401 should notcontain loose metal-to-metal contacts, and should be installable in away that prevents loose metal-to-metal contacts from being formed.Preferably, RF barrier 401 can be cut in the field to custom shapeswithout generating burrs that could touch other metal surfaces andgenerate PIM.

In cases where the RF barrier is applied to existing rooftop surfaces,it is desirable that RF barrier 401 and any mounting system for RFbarrier 401 not penetrate the existing roof membrane, so that anywarranty of the existing roof is not voided. In addition, penetrationsare preferably avoided because they are typically made with metalfasteners, which could become PIM sources themselves.

Preferably, the materials of RF barrier 401 provide good environmentalstability and durability, similar to typical roofing materials. Forexample, RF barrier 401 may provide up to 10 years or more of operatinglife when installed in a rooftop environment. This includes exposure toultraviolet radiation, wind, rain, snow, extreme heat and extreme cold.The material of RF barrier 401 preferably will not rot, support fungalgrowth or delaminate after long term environmental exposure. Inaddition, an RF barrier 401 applied in an area subject to foot trafficshould be able to withstand people walking on the barrier withoutphysical damage or deterioration in performance.

FIG. 7 illustrates a PIM barrier membrane 700, in accordance withembodiments of the invention. Example membrane 700 includes a metallayer 701 sandwiched between two polymeric layers 702 and 703. Metallayer 701 may include, for example, a layer of aluminum having athickness of 10 to 100 nanometers. Other metals may be used, for examplecopper, and other thicknesses of metal may be used if desired. Forexample, in other embodiments, the thickness of the metal layer may beup to 40 microns or more. The metal of metal layer 701 may be vapordeposited or otherwise coated on a film 704 such as a 0.0005 to 0.025inch thick polyester film or other plastic film for ease of handling. Insome embodiments, the metal layer 701 may be enclosed between twosimilar plastic films 704 and 705. In some embodiments, metal layer 701may be a metal foil, which in turn may be bonded to a reinforcement matmade of polyester, glass, or another suitable material.

Metal layer 701 may be continuous, and formed without folds or otherfeatures that may cause metal-to-metal contact. In other embodiments,the metal layer may be perforated, having very small holes to improveadhesion of the polymeric layers. Any holes should preferably have asize of no more than 1/50^(th) of the wavelength of the lowest operatingfrequency of the cell site.

Polymeric layers 702 and 703 may be made of thermoplastic polyolefin(TPO), ethylene propylene diene monomer (EPDM), polyvinyl chloride(PVC), modified bitumen, or another suitable polymer with sufficientdurability for use on a roof. Polymeric layers 702 and 703 may each havea thickness of between 0.010 and 0.100 inches, or another suitablethickness. Membrane 700 may have an overall thickness of between 0.020and 0.250 inches, or another suitable thickness. In some embodiments,membrane 700 may have an overall thickness of between 0.040 and 0.150inches, and may preferably have a thickness between 0.050 and 0.100inches. Polymeric layers 702 and 703 may be made of the same material orof different materials. For example, polymeric layer 702 could be madeof TPO and polymeric layer 703 of EPDM. Many combinations are possible.

Polymeric layers 702 and 703 may include a fire retardant such asmagnesium hydroxide, calcium carbonate, talc, zinc borate, or anotherfire suitable retardant. Preferably, the fire retardant isnon-halogenated. In some embodiments, the fire retardant may comprise20-40 weight percent of the polymeric layers.

Polymeric layers 702 and 703 may also include various stabilizers suchas high and low molecular weight hindered amine light stabilizers,phosphate antioxidants, phenolic antioxidants, or other suitablestabilizers. In some embodiments, one of polymeric layers 702 and 703may be designated as a top layer of membrane 700, and may include morestabilizer than the bottom layer. For example, top layer 702 may include1-5 weight percent of a stabilizer or stabilizers, while bottom layer703 may include 1-2 percent of a stabilizer or stabilizers. Theformulations of top and bottom layers 702 and 703 need not be identical.For example, top and bottom layers 702 and 703 may use different fireretardants or different stabilizers, or may differ in other respects. Inother embodiments, the formulations of top and bottom layers 702 and 703may be the same.

For the purposes of this disclosure, the words “top” and “bottom” referto the orientations of the various materials in the figures. While theseorientations correspond to orientation of the materials as they would beinstalled on a rooftop, it is to be recognized that the materials may beinstalled in other orientations, for example on a vertical wall, andthat the claims are intended to encompass these other orientations.

FIG. 8 illustrates a process of making a PIM barrier membrane such asmembrane 700, in accordance with embodiments of the invention. Metallayer 701 and its support film or films 704 and 705 are paid off of asupply roll 801, and passed between calendering rollers 804 and 805.Extruders 802 and 803 inject the material of polymeric layers 702 and703. The completed membrane 700 is wound into a roll 806 for storage andshipment to a job site.

The process shown in FIG. 8 is known as co-extrusion, because bothlayers 702 and 703 are formed at the same or nearly the same time. Inother embodiments, only one of layers 702 or 703 may be formed in afirst pass. The half-completed membrane may then be sent through asecond extrusion pass to form the other polymeric layer.

The extruder of FIG. 8 may have a length to diameter ration of 40:1,50:1, 60:1, or another usable ratio. The components of polymeric layers702 and 703 may be fed into extruders 802 and 803 in neat orpre-pelletized form. Each extruder may include are extruder screw andtransfer zones that are optimized to mix the formulation components.From the extruder, the formulation can go through a melt pump whichprevents pressure fluctuation of the melt going into the die. The lip isused to control the amount of formulation being applied to the metallicfilm.

Calendering rollers 804 and 805 form a calendering stack, and may bestainless or treated galvanized rollers. More than two rollers may beused, for example four rollers. The thickness of membrane 700 iscontrolled by the die Up and the gap or opening of the calenderingrollers. The gap or pressure generated from the calendering rollershelps control adhesion of the polymeric layers to the metallic film.

Completed membrane 700 may be considered a “single ply” membrane, eventhough it comprises various layers. In the field of roofing, the term“single ply” distinguishes a roofing membrane such as membrane 700 froma “built up roof”, in which multiple components such as asphalt andballast are separately applied to the roof. Membrane 700 may be suppliedin any workable size, for example in rolls up to 50 feet wide or moreand 100 feet long or more. In some embodiments, membrane 700 may beprovided in rolls about 6 feet wide.

Metal layer 701 provides the desired RF attenuation in the 500 MHz to6000 MHz frequency range. Plastic film layers 704 and 705 on both sidesof the metal layer 701 provide electrical insulation to prevent unwantedmetal-to-metal contact when the membrane contacts other metal surfacesor is folded back on itself. Because the metal layer is very thin, burrsdo not form when the membrane is cut with a razor blade.

In some embodiments, for example when higher RF attenuation is required,a second layer of aluminum or other metal can be added to the stack. Forexample, FIG. 9 illustrates a PIM barrier membrane 900 in accordancewith other embodiments of the invention. In membrane 900, two metallayers 901 and 902 are interleaved with three plastic film supportlayers 903, 904, and 905. Layers 901-905 are then encapsulated betweenpolymeric layers 906 and 907, for example in an extrusion processsimilar to that described above. Metal layers 901 and 902 may be similarto metal layer 701 described above. For example, metal layers 901 and902 may be made of aluminum, copper, or another suitable metal, and maybe 10 to 100 nanometers thick, or another suitable thickness. Plasticfilm layers 903-905 may be similar to film layers 704 and 705 describedabove, and polymeric layers 906 and 907 may be similar to layers 702 and703 described above. Intermediate plastic film layer 904 prevents thetwo metal layers 901 and 902 from touching each other, so that the twometal layers do not become a PIM source.

In other embodiments, additional shielding may be provided using asingle, thicker layer of metal.

A PIM barrier membrane such as membrane 700 or membrane 900 may beattached to a roof in any suitable way. In some embodiments, the edgesof the PIM barrier membrane are heat welded to the existing roofmembrane. In other embodiments, the PIM barrier membrane may be adheredto the existing roof using an adhesive or using a tape at the edges ofthe membrane. A PIM barrier membrane in accordance with embodiments ofthe invention may also be used in re-roofing applications and in newconstruction.

The polymeric layers of a membrane embodying the invention may be of anyavailable color. For example, top layer 702 or 906 may be colored tomatch an existing roof, either by coloring the material of which thelayer is made to match the existing roof, or by adding a coloring layerto the top of the membrane. In other embodiments, the color may beselected for different reasons. For example, top layer 702 or 906 may bemade white or otherwise highly reflective to reduce the absorption ofheat into the building.

While a membrane such as membrane 700 or membrane 900 is effective inmitigating PIM, other forms of RF barrier may be useful as well, aloneor in conjunction with a barrier such as membrane 700 or membrane 900.For example, a self-adhesive barrier may be helpful for vertical orirregularly shaped surfaces.

FIG. 10 illustrates an exploded view of a self-adhesive RF barrier tape1000 in accordance with embodiments of the invention. Self-adhesive RFbarrier tape 1000 includes a polymeric top layer 1001, made of awaterproof, durable material suitable for exposure to the elements on arooftop. For example, polymeric top layer 1001 may be made of TPOthermoplastic polyolefin (TPO), ethylene propylene diene monomer (EPDM),polyvinyl chloride (PVC), or another suitable material. Polymeric toplayer 1001 may have a thickness of 0.030-0.060 inches, or anothersuitable thickness. The top surface of polymeric top layer 1001 may beof any suitable color, for example black, white, gray, or another color.The top surface may be the natural color of the material of polymerictop layer 1001, or may be painted or otherwise coated to impart adifferent color. A highly reflective color such as white may bepreferable in cool roof applications.

The underside 1002 of polymeric top layer 1001 is coated before assemblywith an adhesive, for example an acrylic adhesive.

Self-adhesive RF barrier tape 1000 further includes a metal layer 1003.Metal layer 1003 may include, for example, a layer of aluminum 10 to 100nanometers thick, vapor deposited on a plastic film (not separatelyshown). Other metals and thicknesses may be used. For example, in otherembodiments the thickness of metal layer 1003 maybe up to 40 microns ormore. In some embodiments, metal layer 1003 may be about 18 micronsthick. The plastic film may be made of polyethylene, polyester,polypropylene, or another suitable material, and may have a thickness of0.0005 to 0.025 inches, or another suitable thickness. In someembodiments, metal layer 701 may be a metal foil, which in turn may bebonded to a reinforcement mat made of polyester, glass, or anothersuitable material.

Self-adhesive RF barrier tape 1000 further includes an adhesive layer1004. Adhesive layer 1004 may be made of an adhesive composition thatcan adhere to at least some common roofing materials. Examples of commonroofing materials may include TPO, EPDM, PVC, modified bitumen, metals,brick, fiberglass, and wood. Adhesive layer 1004 may include a pressuresensitive adhesive. In some embodiments, adhesive layer 1004 is made ofbitumen (asphalt) or modified bitumen and has a thickness of between0.030 and 0.060 inches. Other adhesives and thicknesses may be used aswell.

A release liner 1005 is preferably temporarily attached to the bottomsurface of adhesive layer 1004. Release liner 1005 may be a thinplastic, a silicone-coated paper, or another material that lightlyadheres to the adhesive of adhesive layer 1004, but can be easilyremoved without damaging adhesive layer 1004. Self-adhesive RF barriertape 1000 may be shipped to a job site with release liner 1005 in place,and release liner 1005 is then removed just prior to installation oftape 1000.

FIG. 11 shows an edge view of self-adhesive RF barrier tape 1000, in itsassembled state. In order from the top, self-adhesive RF barrier tape1000 includes polymeric top layer 1001, and a layer of adhesive 1101 onthe underside of polymeric top layer 1001. Metal layer 1003 is adheredto adhesive 1101, and is supported by plastic film layer 1102. Adhesivelayer 1004 is adhered to plastic film layer 1102, and is covered on itsbottom side by release liner 1005. In other embodiments, the positioningof metal layer 1003 and plastic film layer 1102 may be reversed.

Self-adhesive RF barrier tape 1000 may be manufactured in any workablesize, for example in rolls 60 inches wide or more. Narrower tapes may bemade, or may be cut from wider tapes. For example, widths of 6, 12, 18,or 24 inches may be produced, or other widths.

Prior to attaching self-adhesive RF barrier tape 1000 to a rooftopsurface, the roof surface should be clean, dry and free of oils orgrease. Dusty surfaces may be primed with a water or solvent basedprimer prior to applying the RF barrier to improve adhesion.Self-adhesive RF barrier tape 1000 should be un-rolled and the pressuresensitive adhesive side pressed against the surface to be covered. Aroller can be used to help press the adhesive into the roof surface tohelp maximize adhesion.

Referring again to FIG. 3, PIM sources such as sources 306 may not beapparent from a visual inspection of roof 201, because they may beburied within roof 201. The presence of PIM at a particular cell sitemay be suggested by a statistical analysis of the performance of thecell site. For example, a cell site with significant PIM may have ashorter average call duration or a higher percentage of dropped callsthan another similarly-situated site. If the differences in performanceare not explained by other factors, PIM may be suspected.

It is desirable to have a method of finding and mitigating PIM sources,in accordance with embodiments of the invention. Such a method may bepracticed when PIM is specifically suspected, or as a matter of generalpractice for network optimization.

FIG. 12 illustrates a technique for locating PIM sources, in accordancewith embodiments of the invention, in the context of building 100 andcell site 200. Two signals of known frequency are injected into antenna301, which produces radio frequency (RF) radiation 1201 containing thetwo known frequencies toward part of roof 201. The RF radiation excitespassive intermodulation (PIM) sources 306, which re-radiate broadspectrum RF radiation.

An intermodulation frequency is calculated from the known frequencies ofthe two injected signals. Preferably, this is the third-orderintermodulation frequency IM3, but other frequencies could be used.

A worker 1202 “walks” the roof area of interest, carrying a probe 1203sensitive to the RF radiation produced by PIM sources 306. Probe 1203 ispassed over the roof and receives intermodulation frequencies generatedby PIM sources generated at locations on the roof. Probe 1203 may be,for example, a PIMHunter™ probe manufactured by Anritsu Corporation ofKanagawa, Japan, or another suitable kind of probe.

The signal received by probe 1203 is fed to a spectrum analyzer 1204. Aspectrum analyzer is an instrument that can separate an electronicsignal into its frequency components, and measure the amplitude of thesignal as a function of frequency. Spectrum analyzer 1204 may be, forexample, a model MS2720T Spectrum Master™ spectrum analyzer manufacturedby Anritsu Corporation, or another suitable kind of spectrum analyzer.

Worker 1202 sweeps the area of interest while monitoring the output ofspectrum analyzer 1204. Preferably, at least the area of roof 201 withinthe half power beamwidth Θ of antenna 301 is swept. Worker 1202 monitorsthe output of spectrum analyzer 1204 to see if the amplitude of anysensed signal at the intermodulation frequency exceeds a predeterminedthreshold. For example, worker 1202 may monitor a graphical display ofspectrum analyzer 1204. Depending on the model of spectrum analyzerused, spectrum analyzer 1204 may be able to generate an audible signalwhen the threshold is exceeded at the frequency of interest.

When the threshold amplitude is exceeded, it may be suspected that a PIMsource has been detected. The roof location may be marked formitigation.

Once potential PIM sources such as sources 306 have been located, eachpotential source can be covered with a radio frequency barrier materialconfigured to mitigate passive intermodulation. FIG. 13 shows suspectedPIM sources 306 covered with respective pieces of barrier material 1301,in accordance with embodiments of the invention. Barrier material 1301may be temporary and put in place for additional testing andverification, or may be a permanent application.

Suitable barrier materials may include absorptive materials such as RFabsorbing foams, or reflective materials such as metal foil encapsulatedbetween nonconductive insulation, for example polymeric layers asdescribed above.

Roof 201 may be re-measured with barrier materials 1301 in place to testwhether the PIM has been satisfactorily mitigated. Re-measurement mayverify a reduction in PIM caused by PIM sources 306, or may reveal thatadditional mitigation measures are needed. For example, there-measurement may detect additional PIM sources that were notpreviously detected, as PIM is a dynamic phenomenon and may be affectedby foot traffic on roof 201, nearby metal objects, wind, or otherfactors. Any additional PIM sources may be mitigated similarly to PIMsources 306. Any mitigated PIM sources that still radiate significantlyat the intermodulation frequency may have additional layers of barriermaterial 1301 applied. Any number of test-and-mitigate iterations may beperformed.

If barrier materials 1301 are temporary, they are preferably removed andreplaced with more permanent materials, for example barrier membraneshaving a metal layer encapsulated between polymeric layers, as describedabove. The barrier material may be self-adhesive, may be adhered to theexisting roof with an added adhesive or by heat welding, or may beaffixed to the roof in another suitable way. Preferably, the existingroof is not damaged. For example, the barrier material is preferably notattached using penetrating fasteners.

The relatively small pieces of barrier material 1301 shown in FIG. 13may be called “walkpads.” In other embodiments, larger areas of roof201, possibly including multiple PIM sources, may be covered with PIMbarrier material, rather than covering the PIM sources individually.FIG. 14 illustrates larger pieces of barrier material 1401 applied tothe entire area of interest on roof 201. Barrier material 1401 ispreferably supplied in a roll, so that installation of larger areas maybe simplified.

In applications where PIM mitigation would be desirable but the buildingowner does not want the RF barrier permanently attached to the roofsurface, a “mat” RF barrier 1500 can be deployed as shown in FIG. 15.The mat may be constructed using the same rugged outer layer 1501 andinsulated metal layer 1502 as self-adhesive RF barrier tape 1000described above, or similar materials suitable for exposure to theelements on a roof. However, the bottom layer 1503 in this form is athick rubber mat such as is commonly used for lining gym floors or horsestalls. Additional layers such as adhesive layers, film backing formetal layer 1502, or other layers may be present. The mat material 1503may have a thickness of 0.5 to 0.75 inches, or another suitablethickness. The weight of the rubber mat provides ballast to hold the RFbarrier in place. Additional ballast in the form of concrete blocks canbe applied to the top of the mat if desired or necessary, for example inareas with higher wind requirements.

Such mat type RF barriers may be supplied in square sections measuring36 inches by 36 inches, or in another suitable shape and size. Largersections are possible to manufacture but are not practical fortransporting onto rooftops due to their size and weight. To cover largesurface areas, multiple mats can be deployed and mechanically joinedtogether as shown in FIG. 16. To prevent RF energy from passing throughthe gap between matts and exciting PIM sources below, self-adhesive RFbarrier material 1000 can be applied over the seams. While this tapedoes not create a continuous electrical connection between theindividual barriers, it does provide sufficient capacitive couplingbetween barriers provide a continuous RF barrier. Additional matconnector mechanisms 1601 may be provided if desired.

An alternate method for producing the mat style RF barrier material 1700is shown in FIG. 17. In this configuration, the outer rugged layer 1701is replaced with a 0.25 inch to 0.38 inch rubber mat layer and the lowermat layer 1703 is reduced in thickness to 0.25 inch to 0.38 inch,yielding the same over-all mat thickness. Metal layer 1702 isencapsulated between mat layers 1701 and 1703. Other layers may bepresent as well.

In some cases, it may be desirable to replace an existing roof membranewith an RF barrier membrane rather than apply separate RF barriers ontop of the existing membrane. This might be the case when an RF barrierneeds to be applied to reduce PIM, but the existing roofing system isclose to the end of its useful life. For this application, an RF barrier1800 as shown in FIG. 18 may be used. RF barrier 1800 comprises an outerrugged layer 1801 with inner insulated metal layer 1802 as previouslydescribed with a second outer rugged layer 1803. The thicknesses of theouter rugged layers may be reduced to 0.015 inch to 0.030 inch per layerto maintain the overall membrane thickness. Outer layers 1801 and 1803may be made of, for example, EPDM, TPO, or PVC. Installation of thismembrane may utilize the existing installation processes and bondingmaterials. The self-adhesive tape RF barrier material 1000 as shown inFIG. 10 can be used at the seams to achieve a continuous RF barrier, asshown in FIG. 19.

The invention has now been described in detail for the purposes ofclarity and understanding. However, those skilled in the art willappreciate that certain changes and modifications may be practicedwithin the scope of the appended claims.

What is claimed is:
 1. A method of mitigating passive intermodulation,the method comprising: injecting two signals of two known frequenciesinto an antenna system, such that the antenna system produces radiofrequency radiation containing the two known frequencies directed towardan area to be tested for passive intermodulation; calculating anintermodulation frequency from the two known frequencies; connecting aprobe to a spectrum analyzer, the spectrum analyzer configured toreceive radio frequency signals produced at the intermodulationfrequency; passing the probe over the area to be tested whilesimultaneously monitoring the signal received by the spectrum analyzer;identifying a suspected source of passive intermodulation when thespectrum analyzer detects a signal amplitude above a threshold value atthe intermodulation frequency; and covering the suspected source ofpassive intermodulation with a radio frequency barrier materialconfigured to mitigate passive intermodulation.
 2. The method of claim1, wherein the radio frequency barrier material is a temporary radiofrequency barrier material, and the method further comprises:re-measuring the signal amplitude at the suspected source of passiveintermodulation at the intermodulation frequency with the temporaryradio frequency barrier material in place, to verify a reduction inpassive intermodulation; and replacing the temporary radio frequencybarrier material with a permanent radio frequency barrier material. 3.The method of claim 1, wherein the radio frequency barrier material is aroofing membrane configured to perform as a radio frequency barrier. 4.The method of claim 3, wherein the roofing membrane comprises: a firstpolymeric layer; a second polymeric layer; and a metal layerencapsulated between the first and second polymeric layers.
 5. Themethod of claim 4, wherein the metal layer is deposited on a plasticsheet, and the metal layer and the plastic sheet are encapsulatedbetween the first and second polymeric layers.
 6. The method of claim 4,wherein at least one of the first and second polymeric layers comprisesthermoplastic polyolefin (TPO).
 7. The method of claim 4, wherein atleast one of the first and second polymeric layers comprises ethylenepropylene diene monomer (EPDM).
 8. The method of claim 4, wherein atleast one of the first and second polymeric layers comprises polyvinylchloride (PVC).
 9. The method of claim 4, wherein at least one of thefirst and second polymeric layers comprises modified bitumen.
 10. Themethod of claim 4, wherein the roofing membrane is self-adhesive. 11.The method of claim 4, wherein the roofing membrane has an overallthickness of between 0.020 and 0.25 inches.
 12. The method of claim 1,wherein the radio frequency barrier material is a walk pad.
 13. Themethod of claim 1, wherein the area to be tested for passiveintermodulation is on a rooftop.
 14. The method of claim 1, wherein theradio frequency barrier material is a radio frequency barrier mat, theradio frequency barrier mat comprising a metal-containing layer joinedto a rubber mat layer.
 15. The method of claim 14, wherein the radiofrequency barrier mat has an overall thickness of 0.5 to 0.75 inches.16. A membrane for reducing passive intermodulation, comprising: a firstpolymeric layer; a second polymeric layer; and a metal layerencapsulated between the first and second polymeric layers; wherein thefirst and second polymeric layers comprise one or more materialsselected from the group of materials consisting of thermoplasticpolyolefin (TPO), ethylene propylene diene monomer (EPDM), and polyvinylchloride (PVC); wherein the membrane is a roofing membrane; and whereinat least one of the first polymeric layer and the second polymeric layercomprises a fire retardant.
 17. The membrane of claim 16, wherein themetal layer is a layer of aluminum.
 18. The membrane of claim 17,wherein the layer of aluminum has a thickness between 10 and 100nanometers.
 19. The membrane of claim 16, wherein the metal layer has athickness of at least 1 micron.
 20. The membrane of claim 16, whereinthe metal layer comprises aluminum vapor deposited on a film, andwherein the aluminum and the film are encapsulated between the first andsecond polymeric layers.